A load device can be used to provide a voltage at the input node of the load device, corresponding to an unknown current level supplied to the load device. Similarly, a load device can be designed to provide this voltage level for an input current on each of two input leads. A load device with two input leads can be coupled downstream of a two-output differential amplifier, such as a differential transconductance amplifier. Additionally, a two-input load device can be configured to provide equivalent voltage levels for equivalent input current levels, e.g. common-mode operation, and to provide differential voltage levels for differential input current levels, e.g. differential-mode operation. These voltage levels generated by the load device can be subsequently processed by downstream devices, such as amplifiers.
A conventional load device with external control circuitry is shown in prior art FIG. 1A. Conventional load device 107 includes a first transistor 123 coupled to a first lead, lead A 103, and a second transistor 117 coupled to a second lead, lead B 105. Gate 123a for first transistor 123 and gate 117a for second transistor 117 are both coupled to a separate control, or bias, circuit 105. The conventional control circuit 105 shown in FIG. 1A uses a single transistor 111 to generate a voltage for gates 123a and 117a from a given input bias current IC 111. However, the prior art circuitry 105 used to bias the load device is external from the load device 107 and can be complicated. Furthermore, the actual level of the current supplied to the load device, e.g. current IA 141 for input lead A 103 and current IB 131 for input lead B 105, is not defined in most applications and can vary significantly. Because bias current IC 111 is not sensitive to the current level supplied to the load device, the biasing can result in undesirable qualities, such as variable operating point, as illustrated in a subsequent figure. Hence, a need arises for a load device that has a control circuit that is less complicated and is tied to the input current to the load device so as to better regulate the operating, or bias, point of the load device.
A graph of the differential impedance versus bias current for a conventional load device is shown in prior art FIG. 1B. The abscissa of graph 100b represents the bias current, shown in microamps, while the ordinate of graph 100b represents the differential impedance in kilo-ohms. Regarding prior art FIG. 1A, IC 111 represents the bias current, while input current IA 141 is equivalent to IC 111 plus differential current .DELTA.I, and input current IB 131 is equivalent to IC 111 minus differential current .DELTA.I. Consequently, the impedance for differential operation is equivalent to the change in differential voltage, .DELTA.V.sub.A-B, measured between input lead A103 and input lead B 105, divided by the change in the differential current .DELTA.I, e.g. Z=[[.delta.(.DELTA.V.sub.A-B ]/[.delta.(.DELTA.I)]]. Graph 100b represents the performance for a 10.mu. transistor width for each transistor in the load device 107 shown in prior art FIG. 1A. In differential-mode operation, the load device should produce different voltage levels on each input to a load device to reflect the different current levels being fed to the load device. The differential impedance of the load device is the mechanism that generates the differential voltage. The greater the differential voltage, the greater the gain of the system. For improved performance, a need arises for a load device with higher differential impedance.
One prior art load device provided differential impedance by making a gate of one of its transistors sensitive to the input voltage. However, this prior art configuration generated a voltage mismatch because the current levels consumed by the load device for each of the two input leads were different. The first lead had a current different from the second lead because it was the only lead that supplied current to specific types of components within the load device. Thus, this configuration did not provide both the true and complementary versions of the differential voltage levels which are very useful to downstream circuitry. Consequently, a need arises for a load device that has true current matching on both inputs, thereby preserving both the true and complement versions of the voltage differential.
A graph of the common-mode impedance vs. bias current for a conventional load device is shown in prior art FIG. 1C. Graph 100c represents the performance for a 10.mu. transistor width for each transistor in the load device 107 shown in prior art FIG. 1A. In common-mode operation, the load device should yield an equivalent voltage on the two inputs of the load device to reflect the equal current being fed to the two inputs. The abscissa of graph 100c represents the bias current, shown in microamps, while the ordinate of graph 100c represents the common-mode impedance in kilo-ohms. Referring to prior art FIG. 1A, IC 111 represents the bias current, while input current IA 141 is equivalent to IC 111 plus differential current .DELTA.I, and input current IB 131 is also equivalent to IC 111 plus differential current .DELTA.I. Consequently, the impedance for common-mode operation is equivalent to the change in voltage, V, for either lead A 103 or lead B 105, divided by the change in the differential current, e.g. Z=[[.delta.V.sub.A ]/[.delta.(.DELTA.I)]].
The common-mode impedance of a conventional load device is excessively high, due partially to the conventional external biasing, such as that shown in prior art FIG. 1A. The external biasing on input leads 103 and 105 translates into a large change in the drain-to-source voltage, V.sub.DS, of the transistors in the load device 107 given a small increase or decrease from the saturation-level of current on input lead A 103 and input lead B 105. Consequently, the voltage level, generated by the load device for the two inputs, varies significantly, albeit evenly, for common-mode input currents. However, a large variation in voltage levels for common mode input might require downstream hardware to be more robust, and hence more costly. Thus, a need arises for a load device that will regulate its loading based on the voltage of the inputs to the load device, so as to provide a narrow range of voltage levels for common-mode operation.
One prior art solution to wide voltage ranges for common-mode operation, uses a voltage-sensitive device, such as a resistor or a diode, to limit the voltage swing. However, by using a voltage-sensitive device, the gain of the load device is compromised. As a result, a need arises for a load device that maintains a maximum gain while providing a voltage-sensitive load device having a stable operating point, and thus reasonably small variations in output voltage for common-mode operation.
In summary, a need arises for a load device with a control circuit that is less complicated and is tied to the input current of the load device so as to better regulate the operating, or bias, point of the load device. Additionally, for improved performance, a need arises for a load device with higher differential impedance. Furthermore, a need arises for a load device that has true current matching on both inputs, thereby preserving both the true and complement versions of the voltage differential. A need also arises for a load device that will regulate its loading based on the voltage of the inputs to the load device, so as to provide a narrow range of voltage levels for common mode operation. Also, a need arises for a load device that maintains a maximum gain while providing a voltage-sensitive load device having a stable operating point.